1. Field of the Invention
The present invention relates to a laser diode module connectable to an optical fiber for optical coupling and used for optical communication, optical measurement or the like.
2. Description of Prior Art
In a photosensor module used for optical communication or the like, efficient optical coupling is necessary between a light emitting element or a detector and an optical fiber. A laser diode module, one of the photosensor modules, has a laser diode, a focusing lens, a lens holder and a ferrule holder for holding an optical fiber. An end of the optical fiber is inserted into the ferrule holder and fixed therein, and a laser beam emitted from the laser diode transmits through the focusing lens so as to be focused on the incident end of the optical fiber.
An aspheric lens made of glass is used mainly as the focusing lens. Recently, an aspheric lens made of plastics is also used as the focusing lens in order to decrease the cost thereof (for example, Japanese Patent laid open Publications No. 5-60952/1993, 61-245594/1986, 5-27140 and 5-60940).
When an aspheric focusing lens is used in a laser diode module, if the aspheric lens is made of glass, it is expensive, and this increases the cost of laser diode module. On the other hand, an aspheric focusing lens made of plastics has a larger thermal expansion coefficient than that of glass, and the index of refraction depends on temperature. Then, when a plastic aspheric lens is used, if a distance between the laser diode and the lens is fixed, the focal distance or a focus position in the direction of the optical fiber is changed with temperature. For example, in an optical system wherein the lens material is amorphous polyorefin, the laser diode emits a laser beam of 780 nm, the distance between the laser and the optical fiber is 10 mm and the magnifying power of lens is three, the shift of the focal position due to temperature is 8 .mu.m/deg. Thus, the coupling efficiency of the laser diode module to the optical fiber changes with temperature because the laser diode, the lens and the optical fiber are all fixed in the laser diode module. The laser diode module is adjusted and fabricated at room temperature, and the output from the laser diode module decreases at high temperatures and at low temperatures. FIG. 7 shows an example of the temperature dependence of the output from the optical fiber between -20 and 80.degree. C. in a prior art laser diode module coupled to a multi-mode optical fiber of core diameter of 50 .mu.m in the above-mentioned conditions. It is found that the output is decreased more than 10% at high and low temperatures.
In order to keep the optical output from a laser diode module below a predetermined reference value for safety purposes during handling, it is known to stop light emission automatically or to limit the optical output when the optical fiber is slipped out from the module. A function which has to be added for controlling the intensity of the laser beam for the safety standard makes the structure of the laser diode module complex and makes the cost thereof higher. A control circuit is needed to stop light emission when slip out of the optical fiber is detected.
In order to limit optical intensity, it is known to provide an aperture member to limit the light intensity, and the aperture member limits the laser beam transmitted through the focusing lens. However, because this restricts transmittance through the area of the aperture, the characteristic is changed due to the scattering of the light emission angle of laser beam. Thus, high precision is needed for fabricating the aperture member. Further, the number of fabrication steps is increased for the position control and the like. These accompany an increase in cost.